This invention relates to conveyor belt scraper blades, and particularly blades for the scrapping and cleaning of high speed coal conveyor belts. It is particularly directed to a reversible scraper blade.
Through the years, in many industries the demands on belt scrapers remained reasonably modest. As an example, rubber compounds as scrapping elements have proven successful when operated with belt scrapers as used in bulk material conveyor installations in which the conveyor belts are, for the most part, relatively short i.e., under 1,000 ft. long and operate at relatively low speeds, i.e. at or under about 500 feet per minute. Usually such belts are mostly vulcanized and therefor do not employ metal splicing devices.
Apparatus particularly successfully designed for this purpose include the belt scrapers shown in co-owned U.S. Pat. No. 4,202,394 issued Sep. 6, 1983 and U.S. Pat. No. 4,696,388 issued Sep. 29, 1989. The scraper blades, as shown in these patents, are reversible i.e., they can be rotated without the necessity of having to remove the scraper blade from its position underneath the belt, to extend the wear life by utilizing an opposite surface of the blade. Also, such blades are symmetrical in that they could be installed in either position, i.e., with either side becoming the leading edge or the trailing edge.
In the 1980's, dramatic changes began to take place in certain bulk conveyor requirements. An example is the introduction into the United States of European long wall coal mining technology. The long wall mining technology enabled U.S. coal mining companies to produce between three to ten times higher tonnage than was conventionally produced at these existing properties.
Dramatic changes in mining techniques required similarly dramatic changes in the belt conveyor systems which removed the material to the surface and beyond. Because of the speed of the mining technology, belt conveyors also increased in speed as well as length. The longer belts often required the employment of metal splices to connect the belt sections. Belt cleaning requirements became severe, with belt speeds of 700 to 900 ft. or more per minute not uncommon.
Increased belt cleaning blade pressures had to be employed, and new materials used to permit the blade to stand up under these conditions, and not be bent backward upon itself in the direction of belt travel. In order to meet the demands of high speeds the blade loading pressures have more than doubled.
Attempts to meet the increased demands on belt scrapers, for example, in the long wall coal industry, have included a consideration and use of urethanes of varying durometers as the scrapping blade wear surface. Some urethanes have proven to be successful when operated with a relatively sharp scrapping edge and at low pressures, in which the edge is positioned to skim or strip the material from the belt. Metal blades have been tried, but a slight misalignment of the blade can result in belt damage.
If urethane is used at high pressure, the heat build up at the interface between the urethane material and the belt, due to friction, can quickly bring the urethane material up to a glass transition temperature, at which point it looses its strength and wears rapidly.
For this reason, the applicant has preferred to employ relatively high pressure, absolutely flat cleaning contact as achieved by applicant's patented pneumatically pressured belt scrapers, exemplified by patent '394 in which a relatively impenetrable contact region is formed between the blade and the belt. This region allows only the belt to pass by, while the extraneous material is scrapped off. The scrapping is accomplished at the front edge or leading edge of the blade, and the remaining thickness provides the required stiffness, support, and wear resistance.
In order to meet the needs of the long wall coal industry, a belt scraper blade for a positive pressure blade support system was designed and placed in use which incorporated a rubber scraper blade made of black or filled molded rubber material of tire tread grade, backed by a semi-rigid cured resin impregnated fiber backing plate. The rubber scraper element, which formed the leading edge, had sufficient resilience and toughness when retained and supported by the fiber backing, so that the rubber scraper blade could be held at about 90.degree. to the belt.
A resin impregnated fiber backing plate has less wear resistance than the rubber. Therefor, the fiber backing plate wears at a more rapid rate than the rubber scrapping element, and full contact pressures through the rubber is maintained.
A double sided blade has been made, as illustrated in FIGS. 1 and 2 and described in greater detail below, but this has required the removal and the end-to-end reorientation of the blade, to present the opposite working surface to the belt. For example, a 60 inch blade may weigh 75 pounds. Reversal requires the awkward handling of the blade from a position which may be below the belt, or an awkward position alongside the belt, and introduced an element of risk to the persons responsible for removing the blade, reversing its position, and reinstalling the same. The procedure is time consuming and at times it is both confusing and difficult, since scrapers are often in relatively inaccessible locations, with poor lighting, thereby at times making it possible for the scraper to be improperly oriented such that the fiber side is leading rather than trailing, or the blade may be accidentally dropped into a waste pit.
There is therefor a need for a high strength, fiber backed, high pressure scraper blade which is integrally formed in a single blade holder or frame, which provides the required wear resistance and strength, and which can be reversed, in set to and in place, simply by rotating the blade through 180.degree. and reattaching the same to the blade support arms. Such a blade does not require personnel to reach extensively under or around a belt in order to present a renewal surface to the belt.